Features of proteins which confer specificity in interaction with other proteins, small molecular weight ligands, or macromolecules are of central importance for regulatory processes in biology. The human glycoprotein, vitronectin, interacts with many macromolecules that are present either in circulation or in the extracellular matrix to provide control of several diverse physiological processes. From the many types of specific interactions that have been observed for vitronectin, the protein is thought to play a role in control of coagulation, regulation of the immune response, control of fibrinolysis, regulation of pericellular proteolysis, and cell attachment. Vitronectin exists predominantly in human plasma in a "folded" or "closed" form which is stabilized by intramolecular ionic interactions, but several external factors can convert the protein to an "extended or "open" form. Additionally, the protein occurs normally in the circulation as a mixture of a single-chain form and a two-chain, disulfide cross-linked protein. Both the transition of the protein from a closed to open form and the proteolytic processing of the protein from a one-to two-chain form have been observed to affect binding of different macromolecules in various ways. With the long-range goal of better understanding factors affecting protein conformation and the nature of protein-ligand and protein-protein interactions, especially in relation to physiological regulatory mechanisms, specific questions concerning vitronectin are posed for my research. How does conversion of vitronectin from one form to another enable it to bind many diverse ligands? Can the conversion of vitronectin from a folded to an extended form be characterized by physical approaches? What are the regions of the protein which interact and stabilize the folded form of the protein? What are the structural requirements for interaction of vitronectin with macromolecules that have effects on hemostasis and pericellular proteolysis? Differential scanning calorimetry, spectroscopic techniques, and hydrodynamic methods will be used to provide a physical basis for the many diverse phenomena associated with the protein. Also, a powerful complementary approach to answering these and related questions will be provided by utilizing site- directed mutagenesis to target amino acid replacements which will define specific structural requirements for certain macromolecular interactions. Expression of vitronectin in a recombinant form is a necessary first step toward using molecular biological approaches to address the questions of interest with regard to this fascinating regulatory biomolecule.